There are many patents disclosing applied techniques in connection with laser beam printer (LBP). Examples of such patents include U.S. Pat. No. 5,128,795, U.S. Pat. No. 5,162,938, U.S. Pat. No. 5,329,399, U.S. Pat. No. 5,710,654, U.S. Pat. No. 5,757,533, U.S. Pat. No. 5,619,362, U.S. Pat. No. 5,721,631, U.S. Pat. No. 5,553,729, U.S. Pat. No. 5,111,219, U.S. Pat. No. 5,995,131, and Japanese patent Nos. 4-50908 and 5-45580. Most of the laser beam printers disclosed in these patents include a laser scanning unit (LSU) that uses a polygonal mirror, such as a quadrigonal or a hexagonal mirror, rotating at a speed as high as, for example, 40000/min, so as to control the laser beam scanning in the laser beam printer.
A conventional laser scanning unit 1 will now be described with reference to FIGS. 1, 1A, and 1B to explain the structure and optic path in general laser scanning units. As can be seen from FIG. 1, the laser scanning unit 1 includes a semiconductor laser 10 that serves as a light source to emit laser beams, which sequentially pass through an aperture 11 and a collimator 12. The laser beams pass through the collimator 12 to form parallel beams and then pass through a cylindrical lens 13, a main function of which is to cause a width of the parallel beams in a sub-major scanning direction or Y-axis to focus in a direction parallel to a major scanning direction or X-axis and thereby form a line image, which is a point in FIG. 1B. The laser scanning unit 1 also includes a polygonal mirror 14 that is adapted to rotate at high speed, so that a plurality of reflection mirrors 15 uniformly and continuously arranged on the polygonal mirror 14 are just located at or in the vicinity of a focal point of the above-mentioned line image. The polygonal mirror 14 serves to control a direction in which the laser beams are projected therefrom. The a plurality of continuous reflection mirrors 15 in high rotating speed are adapted to deflect and reflect laser beams incident on the reflection mirrors 15 in a direction parallel to the major scanning direction or X-axis to an fθ lens 16 at uniform angular velocity. The fθ lens 16 is located at one side of the polygonal mirror 14 and may be a single-element scanning lens, as shown in FIG. 1, or a two-element scanning lens, as that shown in the figures of U.S. Pat. No. 5,995,131. Laser beams incident on the fθ lens 16 via the reflection mirrors 15 on the polygonal mirror 14 are focused to form a circular light spot that is then projected onto a photoreceptor drum 17 to achieve a required scanning linearity. The above-described conventional laser scanning unit has the following problems in use:    1. The rotary polygonal mirror 14 in the conventional laser scanning unit 1 is very difficult to make and requires high manufacturing cost to increase the cost of the laser scanning unit.    2. Since the conventional polygonal mirror 14 must be able to rotate at a speed as high as, for example, 40000/min, and have high precision, the reflection mirrors 15 on the polygonal mirror 14 usually have a very small mirror width in the direction of Y-axis. Therefore, it is necessary to additionally provide a cylindrical lens 13 in the conventional laser scanning unit, so that laser beams passed through the cylindrical lens 13 are focused to form a line (or a point on Y-axis) before being projected onto the reflection mirrors 15 of the polygonal mirror 14. The conventional laser scanning unit therefore has increased number of elements and requires increased assembling operations.    3. The conventional polygonal mirror 14 rotate at high rotating speed, such as 40000/min, to produce relatively high noises. Moreover, a relatively long waiting period is required for the polygonal mirror 14 to reach a working rotational speed. That is, a long waiting period is needed after the laser scanning unit is turned on.    4. According to the assembling structure of the conventional laser scanning unit, laser beams projected onto the reflection mirrors 15 of the polygonal mirror 14 have a central axis that is not aligned with a central rotation axis of the polygonal mirror 14. When designing the fθ lens 16, it is necessary to consider a deviation from the axis of the polygonal mirror 14. Thus, it is more difficult to design and manufacture the fθ lens 16.